This invention pertains to fuel cells and, in particular, to multi-layer electrolyte carrying matrices for use in such cells.
It is known in the fuel cell art to utilize a matrix construction for carrying the fuel cell electrolyte. To be suitable for this purpose, the matrix must be stable in the electrolyte environment, must exhibit a desired degree of bubble pressure (i.e., must be a suitable gas barrier) and must exhibit high electrolytic conductivity and low electronic conductivity. To meet these requirements, attempts have been made to develop multi-layer matrix assemblies comprised of layers, each of which exhibits one or more desired properties.
U.S. Pat. No. 3,481,737 describes an early attempt at a multi-layer structure in which successive layers of amosite and fiberglass are utilized as a fuel cell matrix. In this case, the fiberglass layers exhibit good gas barrier and wicking properties, but are unstable in the fuel cell electrolyte. The amosite layers, on the other hand, exhibit good wicking and stability properties, but are poor gas barriers. The composite, described as preferably having amosite as the electrode contacting layers and illustrated as two such layers sandwiching a fiberglass layer, is thus alleged to have the good wicking properties of the separate layers, the good stability of the amosite layers and the good bubble pressure of the fiberglass layers. Matrices of this type were found to operate only for a limited time, i.e., for well under 100 hours.
Further attempts have been directed to realizing structures with a reduced number of layers, since increasing the number of layers increases the complexity of the structure as well as increases the electronic conductivity. In U.S. Pat. No. 3,575,718, a two layer matrix is disclosed in which one of the layers comprises carbon and the other layer is carbonless and comprises inert organic compounds. In this matrix structure, the carbon layer provides good gas barrier and electrolytic conductivity properties, while the carbonless layer affords a low electronic conductivity.
A further two layer matrix is described in copending commonly assigned U.S. application Ser. No. 24,416, filed Mar. 27, 1979 now U.S. Pat. No. 4,276,356. The matrix of this application comprises a first layer of silicon carbide and a second layer of carbon material. The latter layer provides the matrix with good gas barrier properties, while the former material affords the layer good electrolytic conductivity and low ionic conductivity.
While the aforesaid two layer matrix has demonstrated superior performance with respect to gas barrier and electronic and electrolytic conductivity properties, production of the composite matrix using customary techniques requires special care and is time-consumming. More specifically, when the matrix composite has been constructed, the addition of the electrolyte has been found to cause an undesirable curling or bending of the composite structure. The composite must therefore be restrained from curling during acid addition or, if allowed to curl, must be flatenned, before disposition between the cell electrodes. As stated, this is a tedious and time-consumming process and undesirable where large scale production is desired.
It is therefore a primary object of the present invention to provide a multi-layer matrix which can be readily produced on a large scale.
It is a further object of the present invention to provide a multi-layer matrix which resists curling upon electrolyte addition.